Linewidth Studies in Electron Spin Resonance Spectra : The Para and Ortho Dinitrobenzene Anions

Abstract

Large variations have been found among the linewidths of the different hyperfine lines in the low-temperature electron spin resonance spectra of the p- and o-dinitrobenzene anions generated electrolytically in N,N-dimethylformamide solutions. The magnitude of the variations in the para compound is so great that the spectrum is superficially uninterpretable. Detailed analysis of the spectrum of this radical shows, however, that most of the linewidth differences can be accounted for by modulation through molecular tumbling of the intramolecular anisotropic dipolar and g-tensor interactions. There may also be a small contribution from modulation of the isotropic proton hyperfine splittings, the mechanism that accounts for the alteranting linewidth phenomenon in a number of radicals, but an alternation of the widths is not observed in the p-dinitrobenzene anion spectrum because the contribution from this interaction is small. The sign of the isotropic nitrogen hyperfine splitting aN has been determined by a new method involving only pure dipolar interactions with the nitrogen nuclei and protons and not depending on any assumptions about the magnitudes of the components of the g tensor. A number of features of the relaxation matrix determining the linewidths are discussed, and the problems encountered when the components of a degenerate line have different widths are analyzed. The spectra obtained in dimethylformamide solutions are much better resolved than those previously reported in acetonitrile solutions. Changes of hyperfine splittings with both solvent and temperature are observed.